Cultures of microorganisms are grown among other reasons for purposes of identification, separation, cloning and the accumulation of metabolic products. An early method for growing cultures for one or more of these purposes was to expose the surface of a semi-solid medium known as agar to the bacteria of interest. Because the surface was wet, the bacteria adhered to it, and because nutrients were mixed in the agar, the bacteria in contact with it began to multiply to form a culture. But these localized nutrients were quickly used up so as to limit the amount of growth. Growth was also limited and even altered by the accumulation of metabolic waste in the region of the agar adjacent the colony. Cloning of a particular colony required its identification and removal from the agar by the procedure of carefully accumulating it on a fine probe.
In order to overcome some of the disadvantages of this method of growing pure cultures, vat culture and shaker culture systems were used in which the nutrients and microorganisms were intimately mixed so as to obtain greater growth, but separation of the culture from its metabolic products was obtained by complicated filtering techniques.